Acceleration, i.e., the rate of change of velocity, is a vector that is defined by both direction and magnitude. Typically, the magnitude of acceleration is expressed in meters per second per second (m/s2) or popularly in terms of g-force. A conventional single-axis accelerometer measures acceleration that is directed along an axis with which the single-axis accelerometer is aligned. A conventional tri-axial accelerometer measures acceleration in a three-dimensional space using orthogonally oriented sensors to define the direction of acceleration that is detected. Tri-axial accelerometers can detect acceleration and/or gravity induced reaction forces including vibration, imbalance or shock.
The effects of gravity and acceleration are indistinguishable to an accelerometer. As a consequence, the output of a tri-axial accelerometer has an offset due to gravity. This means that a tri-axial accelerometer at rest on the earth's surface will indicate 1 g along a vertical direction. For the tri-axial accelerometer to measure vertical acceleration due to motion alone there must be an adjustment to compensate for the offset due to gravity. At the same time, there is no adjustment for the tri-axial accelerometer to measure horizontal acceleration due to motion.
Accurate measurement of acceleration depends on the identification of the orientation of the tri-axial accelerator relative to gravity and relative to a device on which the tri-axial accelerometer is mounted. Given that a large number of options are available for mounting a tri-axial accelerometer on a device, the potential for inaccurately identifying the orientation of the tri-axial accelerometer is also large.